Reactive blending, reinforcement and curing of NR/BR/EPDM compounds for tyre sidewall applications

Abstract

Modern tyre sidewall compounds are generally formulated using NR and BR to achieve the required dynamic properties. The most widely used antiozonant, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), is a staining, toxic and environmentally harmful substance. Incorporating EPDM into NR/BR blends improves ozone resistance but leads to poor properties because of differences in filler affinity and cure incompatibility of the rubber components. In this study the cure incompatibility of a commonly used sulfur/CBS system is overcome by using a reactive processing technique. The full amount of curing agents is first added to the EPDM. After a thermal pretreatment step determined by the scorch time of the EPDM, the modified EPDM is mixed with pre-blended NR/BR. The reactive blends show significant improvements in physical properties in comparison to those of conventional EPDM mixes, due to improvements in crosslinking and carbon black distribution. This is confirmed by AFM and TEM images. The physical properties of the reactively mixed NR/BR/EPDM blends are comparable or even superior to those of conventional NR/BR blends for tyre sidewall applications. The blend with 30 phr EPDM has excellent ozone resistance, which means that 6PPD can be omitted completely from the formulation.